Powder coatings are a solvent-free coating system used in a number of applications, including automotive coatings and office furniture. Powder coatings are a type of coating made of a thermosetting resin system. They are applied to a substrate electrostatically and cured at relatively high temperatures. The advantages of powder coatings are that there are no solvents used when they are sprayed, which minimizes the environmental issues related to volatile organic compounds (VOCs). Additionally, any material that does not adhere to the substrate after spraying, known as “overspray,” can be collected and used again resulting in very little waste.
Powder coatings are typically made by first combining resins, a crosslinker, fillers, organic or inorganic pigments, and any other ingredients together in an extruder. The resin is melted and the ingredients are mixed into the resin. The extruded resin is then collected and milled to the required size distribution, usually by impact milling, to make a powder coating. Effect pigments are typically incorporated in subsequent processing steps.
For effect pigments such as aluminum flakes and pearlescent pigments, a significant portion of the color and appearance comes from their structure and shape, which is micron sized platelets with large aspect ratios (pigment width/pigment height). Because of their shape, they are very susceptible to bending and breakage under high shear mixing and have poor dispersion and orientation of the effect pigment in the powder coating. Thus, if they are incorporated during extrusion and/or milling steps, then the effect pigments are damaged and their appearance suffers.
An additional difficulty related to safety. To make any powder coating using effect pigments, a solvent free effect pigment powder must be used, which can cause a large release of dust as it is being dispensed. For pearlescent pigments, this can pose an inhalation hazard to those working with pearlescent pigments and precautions must be made to limit dust exposure. For metallic pigments, the dust not only poses an inhalation hazard, but can also be potentially explosive if clouds of dust are generated. This risk increases with the fineness of the pigments, and means that many converters will outsource the bonding of fine metallic pigments to reduce their risk.